id?

[Johnny Reefer]

Hard to say. Not sure what species you got.
Just reading up on them with a book in front of me here (Calfo & Fenner).
"Voracious algae grazers that are not always useful for the mixed invert aquarium."
"Most are herbivorous, some are predatory..." [Coral grazers. Carribean Orange (Lucapina) and Black Limpet (Scutus)].
Also, this...."Even among the algae grazing species, they can be devastating to desirable algae forms like corallines...."

I'd say keep 'em, but monitor what they are up to!

Cheers,

PS Apparently there are claims of them causing abrasion to acrylic tanks, too.